Electron tube



pr 3 F. MICHELSSEN 2,076,662

ELECTRON TUBE Filed Sep 1 15, 1935 INVENTOR 4 mg m n M .A z Y %B Patent ed A,,.. 13, 1931 T UNITED] STATES ELECTRON TUBE .Fritz Michclssen, Berlin, Germany, assignor to -'.lelefunken' Gcsellschaft fur Drahtlose Telegraphic m. b. 11., Berlin, Germany, a corporation of Germany Application September 13, 1933, Serial No. 689,171

1 a In Germany September 24, 1932 1 Claim.

netic means. In Braun or cathode ray tubes it is thus possible to register upon the luminous or fluorescent screen the curve shape of the deflecting impulsesin rectangular coordinates if the directions of the two fleldsin'projection. upon the screen are at right angles to each other. Arrangements of this kind are .not only used for oscillographic work, but also for television purposes.

However, the arrangements heretofore disclosed and used evidence a number of error sources, especially when the point is to cover comparatively long tracks upon the screen in a relatively short tube with relatively short ray pencils. If the ratio between the pencil or spot track upon the fluorescent screen to the pencil leverage (i. e. distance between the point of action or deflecting means and. the screen) exceeds a certain limit, in other words, if the angle of deflection becomes great in order that in commercially or technically useful tubes of reduced length, for instance, large television pictures may be re-created, defects in the pictures become very troublesome as experiments have demonstrated. These diiiicultiei manifest themselves in the form of expansions or spreading of the point or spot of impingement of the electron beam or pencil upon the screen whenever relatively large deflections are concerned.. This defect becomes especially annoying in high vacuum tubes though it arises also in the case of pencils f ocussed by a gas-filling with or without auxiliary electric or magnetic fields, though only I to a limited degree. lli 'hese defects in re-creation of the picture are traceable to various causes. For instance, one

cause resides in that the pencil leverages fail to.

be the samefor difierent deflection angles and for the two deflecting means in both coordinates in the arrangements usually employed. Ways and means have already been disclosed whereby, with a view to eliminating the said source of defects, the lengths of the pencil figured from the point of action of deflector means to the screen, are rendered the same regardless of deflection. Another source of defects in picture re-creation for which the present invention discloses remedial ways and means follows from this consideration.

5 Referring now to the drawing; in which Fig.1

shows a tube illustrating schematically the beam deflection paths;

Fig.2 shows an optical analogue of an electron beam focusing system; a

Fig. 3 shows one embodiment of my invention;

and

Fig. 4 shows another embodiment of my in- 'vention; a detailed description of my invention will follow.

Fig. 1 shows a usual arrangement wherein numeral l denotes the cathode surrounded by a Wehnelt cylinder 2', 3 is the anode, 4 a pair of deflector plates, and 5 the screen. Between anode and deflector plates there is mounted an annular coil 6 serving as an electron collector or condenser PATENT OFFICE" device, though the focussing action of the said coil could, also be produced by suitable electrical fields. The pencil of electrons diverges considerably between the object to be'imaged (here an anode being diaphragmic in nature) and the coil the magnetic fleldof which'is then so regulated and adjusted that the pencil will pass through the deflector means under convergent condition, and that, in non-deflected state, its node will come to be located upon the screen being of spherical form. If, however, the pencil is deflected it will be impossible in an arrangement" of the said sort to collect. or condensethe pencil-enclosing lines again in a point of suflicientsharpness whenever there occur large changes indirection, even if the screen surface isso' disposed that the pencil node willbe caused to fall therein for all deflections. The more the-convergent pencil is refracted by the deflecting field (the parabolic curving or bending of the pencil occasioned by the electric field is disregarded inthe drawing) the more will thesize of its focus be altered compared'with that of the non-deflected pencil.

' The path of the electron pencil as shown in Fig ."1'couldbe likened to an optical ray path as roughly indicated in Fig. '2. Inasmuch as the electrons are subject to constant acceleration, monochromaticlight has been supposed to be dealt with in'Fig. 2 which issubject to divergence between the slit 1 and the lens 8 whence it traverses a pris'm'9 under convergent condition.

Such an arrangement when turning the prism evidences a'widening of the image of the slit in position 11 compared with position I. ,And in a very similar way the images of. the diaphragm or slit in'a Braun tube show different diameters according to the position of the pencil.

The present invention discloses ways and means whereby, in spite of large angles of deflection, and in spite of considerable divergence and conthe deflection has been produced by means 4.. It is thus feasible to condensethe lines of rays of the already deflected pencil in a sharp focus or node irrespective of the angle of deflection. This scheme is particularly valuable whenever the point is to re-create large pictures of-uniform sharpness with tubes of technically useful or acceptable length, if the pencil or spot, for instance, in the presence of a deflection a of over 20 mm. out of its position of rest, and of a length of pencil b of 320 mm, and less, is to.

possess a constant diameter upon the screen, i. e., when the desideratum is to make the size of the picture independent to a large extent no matter what the changes in direction, in thepresence of a ratio a:b= 20/320= 1/16 or deflection angles 3 degrees.

However, the conditions 0* picture re-creation become still more favorable if, according to Fig. 4, in addition to the condensing or focussing system 6a in the rear of the deflector. means, a

focussing system 6b is disposed also between anode and deflector means. In that case, the condensor action of both focussing parts can be so adjusted that the electron pencil between the diaphragm acting, for instance, as the anode 40 as far as the first condensing system is divergent, is then rendered parallel and in this state caused to pass across the deflecting fields of an electrical or magnetic nature and is then, after deflection, 're-focussed against by the second coil systerm. .It will be, of course, suitable to so form and design the focussing system interposed between defiecting device and picture surface that, optically speaking, it will act like'a lens of very large aperture and yet sufiiciently small focal length in order that the pencil, in spite of any change in its sense of incidence, will be imaged with equal sharpness in all positions. In the construction of such an arrangement, this fact should also be kept in mind that the focussing fields should not act into or trespass upon the spheres of action of the deflecting fields in order that, for instance, re-creation may occur in rectangular coordinates and that the corresponding form of the deflecting force-lines may be produced without any difliculties. 1r recourse is bad to annular coils for the purpose of insuring the 4 aura cos vergence of the path of the ray pencil. inside the I focussin'g between deflector system and screen.

or between deflector system and anode then it will-be suitable-to make their distances from the deflector means not less than one-half of the coil diameter. 1

In the .case of tubes which, for the purpose of concentrating theelectron-ray pencil, are provided with a gaseous atmosphere, this focussing means will act upon the pencil even after the latter has undergone deflection. Having recourse again to, the optical analogy it may be stated that the ionic core tending to focus the pencil will then have an effect as if, together with the deflecting forces, a foeussing means of reduced aperture were partaking of the motion synchronously. However, even in tubes of this kind the disposition of a focussing device between deflector system and screen surface results in an improvement of the picture quality especially when the point is to modulate at the same time the intensity of the deflected pencil.

It might appear as if, as a result of the interposition oi. focussing means as disclosed and illustrated in Figs. 3 and 4 the Braun tube would turn out to be of greater length than the arrangements known in the earlierart as shown in Fig. 1. However, if the known arrangement is to produce pictures of like quality as in the arrangement of this invention, then in Fig. 1 the leverage (lever arm of the pencil must be greatly extended, for producing the same track of the spot upon the screen; in fact, the known arrangement must then be made of such elong-ated'form that, no matter what the size of the picture, the ratio a:b will be=1/ 16. But with the new arrangement of this invention, satisfactory pictures also of larger picture area are realizable even with tubes that are technically more readily constructed.

Having now described my invention,- what is claimed is:

An electronic tube comprising means for developing an electron beam within the'tube and for causing the developed beam to impinge upon a spherically shaped fluorescent end wall of the tube, a pair of electrodes to which voltages are supplied to cause the developed beam to scan the entire activearea of the fluorescent end wall of the tube upon which it impinges, a magnetic focusing coil intermediate the pair of electrodes and the beam developing means, and a second magnetic coil intermediate the pair of electrodes and the fluorescent end wall for focusing the electrons of the beam to a sharply defined spot irrespective of the elemental area of the fluorescent end wall upon which they are instantaneously impinging, said fluorescent end wall having a curvature concave toward the I second magnetic coil and whose radius is the axial distance between the center of said coil and the fluorescent FRITZ lVlICHELSSEN.

end wall. 

